October 24, 2007

Tropical Cyclones of China

Recently, former Vice President Al Gore won a share of the Nobel Peace Prize for his contributions to our understanding of the global warming problem. His film was seen as a masterpiece that certainly sealed the deal on his Nobel Prize. However, on the same day the Nobel committee honored Gore, world renowned hurricane specialist Dr. William Gray told a packed lecture hall at the University of North Carolina that humans were not responsible for the warming of the Earth and not responsible for alter hurricane patterns, as strongly suggested in the Gore film. Gray told the crowd “They’re going to the Gore movie and being fed all this,” and “It’s ridiculous. The human impact on the atmosphere is simply too small to have a major effect on global temperatures.” Dr. Gray said there were 101 hurricanes from 1900 to 1949 in a period of cooler global temperatures compared to 83 hurricanes between 1957 and 2006. Don’t look for Dr. Gray to receive a Nobel Prize anytime soon.

As we sit through a ho-hum year in terms of hurricanes (a.k.a., tropical cyclones) in the Atlantic Ocean, Gulf of Mexico, and Caribbean Sea, another article has appeared in the professional literature with results that call into question any prediction about more tropical cyclones in the future. The latest work comes from a team of scientists with China’s Shanghai Typhoon Institute and is published in the Chinese journal Acta Oceanologica Sinica (you may notice a few misspelled words and odd phrases in some quotes from the article). The work was funded by the National Natural Science Foundation of China and the Special Climate Project of China’s Meteorological Administration.

Li et al. begin their article noting that the ongoing buildup of greenhouse gases has caused global surface air temperatures and sea surface temperatures to increase in the past and will likely lead to more warming in the years and decades to come. They state “tropical cyclone’s genesis, motion and development are greatly influenced and controlled by the large-scale circulations of atmosphere and ocean, so it is reasonable to consider that the climatic change would affect the TC climatological behavior in the future.” They review the literature suggesting that a 1°C increase in sea surface temperatures could increase the maximum wind speed of tropical storms by 3.3% (we have reviewed this issue many times in the past). But, before the greenhouse train gets rolling, they state “It is controversial whether climate models are able to be used to simulate the tropical cyclone’s activities or not.”

Li et al. begin their analyses by examining the number of cyclones in the western North Pacific Ocean from year to year from 1949 to 2003. As seen in Figure 1 below, the number of tropical cyclones has declined substantially from the early 1970s to the present. They state “There are more tropical cyclones generated from the early 1950s to the early 1970s and less tropical cyclones from the mid-1970s to the present.” They further note “less tropical cyclones in El Niño years such as in 1969, 1982 and 1987” and “more tropical cyclones in La Niña years such as in 1964, 1971, 1988 and 1999-2000”. In other words, warm El Niño years produce fewer tropical cyclones in the western North Pacific compared to the cool La Niña years. We have presented similar findings from other parts of the world in past essays.


Figure 1. Number of tropical cyclones in the western North Pacific, including the South China Sea, for the period 1949 – 2003. Solid and dashed lines are for the whole year and the period June to September, respectively (from Li et al., 2007)

Li et al. state that the formation of a tropical cyclone is dependent on a lot more than the temperature of the air and sea. Other important variables include vertical wind shear of atmospheric circulation between the upper and lower troposphere, divergence flow in the upper atmosphere, and thermodynamic instability. Their empirical analyses show that these variables have been significantly related to tropical cyclone formation over the period of historical records and will certainly be important in the future.

To peer into the future, the team selected output from the Geophysical Fluid Dynamics Laboratory (GFDL) climate model widely used by the Intergovernmental Panel on Climate Change (IPCC) under various “scenarios” described by the award-winning IPCC (IPCC recently shared the Nobel Prize with Gore). In analyzing the output, they conclude “the atmospheric general circulation is likely changed into a state which is favorable for the formation of tropical cyclones from the early 2010s to the later 2020s, so there would be more tropical cyclones generated over the western North Pacific over this period.” Before they are awarded a Nobel Prize, they ruin any chances by then stating “Opposite conditions may happen from the early 2030s to the mid-2040s with less tropical cyclones existence.” They conclude “it seems that the long-term trend change of tropical cyclone frequency within the next half century would be less than about 5% decrease,” ultimately finding “It is indicated that in response to the global climate change the general circulation of atmosphere would become unfavorable for the formation of tropical cyclone.”

Gore may have gotten the Prize, but clearly not for telling the full story.

Reference:

Li, Y., X. Wang, R. Yu, and Z. Qin. 2007. Analysis and prognosis of tropical cyclone genesis over the western North Pacific on the background of global warming. Acta Oceanologica Sinica, 26, 23-34.




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